KR20100048074A - Mode detector using nmc(normalized maximum correlation) method, ofdm receiving apparatus comprising the same detector, and method of detecting guard mode using the nmc method - Google Patents

Abstract

PURPOSE: By using the same signal the OFDM(Orthogonal Frequency Division Multiplexing) receiver including the mode detector using the NMC method, and mode detector and the guard mode detecting method using the NMC method detect the FFT(Fast Fourier Transform) mode and guard mode. CONSTITUTION: By using at least, the serially connected memory of a FFT mode detector(100) detects the FFT mode. A guard mode detector(200) detects the guard mode by using the NMC method. The FFT mode detector comprises serially connected memories. The FFT mode detector detects the maximum value by being input the correlation value.

Description

Mode detector using NMC (normalized maximum correlation) method, OFDM receiving apparatus comprising the same detector, and method of detecting guard mode using the NMC method}

The present invention relates to an OFDM receiver, and more particularly, to a mode detector for detecting a mode for an OFDM signal and a mode detection method thereof.

Orthogonal Frequency Division Multiplexing (OFDM), which is recently used as a useful method for high-speed data transmission in a wired / wireless channel, is a method of transmitting data using a plurality of carriers. A method of converting in parallel and modulating and transmitting a plurality of sub-carriers, that is, sub-channels, having orthogonality to each other is transmitted.

Such OFDM schemes include digital / audio broadcasting, digital TV, wireless local area network (WLAN), wireless asynchronous transfer mode (WATM), and broadband wireless access (BWA). It is widely applied to transmission technology. However, due to its hardware complexity, it has not been widely used, but it has been realized by the development of various digital signal processing technologies including a Fast Fourier Transform (FFT) and an Inverse Fast Fourier Transform (IFFT).

The OFDM scheme is similar to the conventional Frequency Division Multiplexing (FDM) method, but above all, it maintains orthogonality among a plurality of subcarriers, thereby achieving optimal transmission efficiency in high-speed data transmission. In addition, it has good frequency efficiency and is strong in multi-path fading. In addition, the OFDM scheme is superimposed on frequency selective fading by using the frequency spectrum superimposed, and it is possible to reduce the influence of intersymbol interference by using a guard interval, and it is possible to simply design the equalizer structure in hardware and to prevent impulsive noise. It has the advantage of being strong.

In such an OFDM scheme, since information to be transmitted on a transmitting side is transmitted on a frequency by FFT inverse transform, demodulation as in a general transmission scheme is possible by FFT transforming a signal received on a receiving side. However, in order to perform FFT conversion on the receiving side, FFT conversion may be performed only by knowing where to FFT conversion of samples of the received signal. That is, the starting point of the data sample to be FFT transformed and the sample interval of the data to be FFT transformed should be known.

However, the OFDM symbol has a different number of samples, that is, FFT length, depending on the transmission mode. In addition, the OFDM symbol includes a guard interval (GI) to prevent inter-symbol interference (ISI). Accordingly, FFT mode detection for detecting the FFT length and guard mode detection for detecting the guard interval are performed.

Currently, the system using the OFDM scheme has several FFT modes and also has several guard modes for each FFT mode. For example, for a DVB-H system, there are three FFT modes of 2k, 4k, and 8k, and four lengths with 1/4, 1/8, 1/16, and 1/32 ratios for each FFT mode. Has a guard mode. Therefore, a total of 12 cases of modes exist. On the other hand, since the DVB-T system has only two FFT modes of 2k and 8k, there are a total of eight modes.

Conventionally, a method of finding a correlation value and finding a maximum value of each mode has been used to detect these modes. However, in the case of performing a method of finding a maximum value by obtaining a correlation value for each FFT mode and each guard mode as in the related art, the detection time increases because the operation needs to be performed 8 or 12 times. Accordingly, a problem of changing channel characteristics may occur. In addition, due to such a change in channel characteristics, the maximum value is changed, which causes a serious problem in that a mode detection cannot be performed. Furthermore, since the conventional mode detection method uses a large amount of memory, the hardware size increases, which is a big obstacle in reducing the size of the receiving device.

The problem to be solved by the present invention is to solve the problems of the conventional mode detection method, to minimize the mode detection time and minimize the use of memory mode detector, OFDM receiver including the mode detector and guard mode detection method To provide.

In order to achieve the above object, the present invention provides an FFT mode detector for detecting the FFT mode using at least two memory devices connected in series; And a guard mode detector for detecting a guard mode using a normalized maximum correlation method.

In the present invention, the NMC method uses the absolute value of the moving sum of the correlation values obtained using the storage device as a numerator, and the correlation value of the OFDM symbol itself before input to the storage device. Using the absolute value of the shifted sum as a denominator, the guard mode can be detected by counting the number of samples whose fractional value is equal to or greater than a predetermined threshold. In order to implement the NMC method, the guard mode detector may include: a MUX that receives and multiplexes correlation values obtained by using the memory device; A first moving sum unit for moving the MUX output; A second shift sum unit for shifting a correlation value of the OFDM symbol itself before input to the storage device; First and second absolute value calculators for calculating absolute values of output values of the first and second moving sums, respectively; A multiplier for multiplying an output value of the second absolute value calculator by a predetermined value; An adder / subtracter for adding or subtracting an output value of the multiplier to an output value of the first absolute value calculator; And a guard mode detector determining a guard mode according to a result of the adder and subtractor.

In addition, in order to achieve the above object, the present invention, FFT mode detection, guard mode detection, STO (Symbol Time Offset), FCFO (Fractional Carrier Frequency Offset) estimation for the OFDM signal converted to the baseband digital signal through the receiver And a pre-FFT unit for performing symbol frequency offset (SFO) estimation. A CP remover for removing a cyclic prefix (CP) for the OFDM symbol; An FFT unit performing an FFT on the output signal of the CP removing unit; A distributed pilot extractor for extracting a distributed pilot with respect to the output signal of the FFT unit; A channel estimating apparatus for estimating a channel using the distributed pilot with respect to the output signal of the FFT unit; A demapper which demaps an output signal of the channel estimator; And a channel decoder for performing channel decoding on the output signal of the demapper, wherein the guard mode detection is provided through a guard mode detector using an NMC method.

Furthermore, in order to achieve the above object, the present invention comprises the steps of determining whether the FFT mode detection is finished; Setting an initial value for guard mode detection; Determining the number of detections; Counting samples via NMC method; And determining each guard mode based on the counted number of samples, and counting each guard mode.

In the present invention, if the FFT mode detection is not finished in the FFT mode detection determination step, the FFT mode detection is performed, and when the end of the FFT mode detection is terminated, proceeds to the initial value setting step, in the initial value setting step, The number of detections may be set to a predetermined number and the number of modes may be set to zero. Further, in the detection number determining step, it is determined whether it is greater than the detection number set in the initial value setting step, and if it is large, the guard mode having the maximum value among the number of guard modes counted up to that time is the guard mode of the received OFDM symbol. It can be estimated as

The counting of the guard mode may include determining whether the number of samples is greater than zero; Comparing the number of samples with a first threshold; Comparing the number of samples with a second or third threshold; And increasing the number of corresponding guard modes based on the result of comparing the second or third threshold values, wherein the first threshold value is an average number of samples in 1/8 mode and 1/16 mode, The second threshold may be the average number of samples in 1/32 mode and 1/16 mode, and the third threshold may be the average number of samples in 1/8 mode and 1/4 mode.

In the mode detector using the NMC method, the OFDM receiver including the mode detector, and the guard mode detection method using the NMC method, the FFT mode and the guard mode can be detected using the same signal. By using the NMC method, fast and accurate guard mode detection is possible.

On the other hand, by detecting the guard mode using the NMC method, the memory used for the conventional guard mode detection is unnecessary, thereby providing an advantage that can significantly reduce the hardware size.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description, when a component is described as being connected to another component, it may be directly connected to another component, but a third component may be interposed therebetween. In addition, in the drawings, the structure or size of each component is exaggerated for convenience and clarity of explanation, and parts irrelevant to the description are omitted. Like numbers refer to like elements in the figures. It is to be understood that the terminology used is for the purpose of describing the present invention only and is not used to limit the scope of the present invention.

1 is a block diagram of an OFDM receiver using an NMC method according to an embodiment of the present invention.

Referring to FIG. 1, the OFDM receiver of the present embodiment includes an RF tuner 100 (RF Rx), an analog digital converter 200 (ADC), a cyclic prefix remover (300), a pre-FFT unit ( 400, Pre-FFT Unit, FFT Unit 500, Pilot Extractor 600, Pilot Estimator, Channel Estimator 700, Demapper 800 and Channel Decoder 900, Channel Decoder ).

The RF tuner 100 converts the received signal into a baseband signal, and the ADC 200 converts an analog signal into a digital signal. The CP remover 300 removes the CP attached to the received signal, and the pre-FFT unit 400 obtains the synchronization for the symbol. That is, the pre-FFT unit 400 estimates FFT mode detection, guard mode detection, symbol timing offset (STO) / fractional carrier frequency offset (FCFO), and symbol frequency offset (SFO).

In the present embodiment, the FFT mode detection and the guard mode detection are integrally performed using the same signal as in the prior art, and in particular, the guard mode detection is detected through a normalized maximum correlation (NMC) method. The description of the FFT mode detector and the guard mode detector for this mode detection will be described in more detail with reference to FIG. 3.

The FFT unit 500 performs an FFT on signals from which the CP has been removed, and the pilot extractor 600 extracts a scattered pilot on the signals on which the FFT has been performed. The channel estimator 700 estimates a channel using the extracted distributed pilot.

After channel estimation, the demapper 800 performs demapping on the data through demodulation in response to the mapped modulation scheme. For example, when a transmission signal is mapped in BPSK (Binary Phase Shift Keying), DBPSK demodulation is performed. In the case where the transmission signal is mapped in QPSK (Quadrature Phase Shift Keying), DQPSK demodulation is performed.

After demapping, the channel decoder 900 performs final channel decoding based on the channel previously estimated by the channel estimating apparatus 700.

In the mode detection, the OFDM receiver can detect the FFT mode and the guard mode by using the same single signal, and in the case of the detection of the guard mode, the guard mode can be quickly and accurately set by using the NMC method. Can be detected. Furthermore, by using the NMC method, the memory used in the conventional guard mode detection is not used and the memory size can be reduced in hardware design.

2 is a structural diagram showing an OFDM symbol structure in a pre-FFT period.

Referring to FIG. 2, an OFDM symbol, such as a DVB-T or DVB-H symbol, copies an end portion of a symbol data period to form a guard period, that is, a guard period GI. Thus, the front part and the back part of the symbol are correlated with each other.

That is, each OFDM symbol is divided into an FFT mode Tu and a GI mode section Tg. The data of the GI mode section is a copy of the data of the last part of the FFT mode section. Due to the structure of the OFDM symbol, if the OFDM symbol is delayed by the FFT mode interval and the correlation value is calculated from the end portion of the OFDM symbol, the peak of the correlation value is obtained from this portion, and the period at which the peak is formed is Since it corresponds to the length of an OFDM symbol, the FFT mode can be detected by measuring the period in which the peak is formed.

Meanwhile, in the conventional case, the FFT mode is detected for each mode. However, in the present embodiment, the FFT mode is directly detected for one identical OFDM signal using a memory device connected in series. The description thereof will be described in more detail with reference to FIG. 3. Thus, there is an advantage that the FFT mode detection can be performed quickly.

3 is a block diagram illustrating an FFT mode detector and a guard mode detector of FIG. 1.

Referring to FIG. 3, the FFT mode detector 100 according to the present embodiment includes four memory devices 110a to 110d, a conjugate complex unit 120, a conjugate complex multiplier 130a to 130c, and a maximum value detector 140. ).

The four storage devices 110a to 110d are connected in series with each other and each have a memory size of 2048 bits (2k). Therefore, the four memory devices 110a to 110d can store the FFT output signal having a symbol size of 0 to 8192 bits. That is, since four memory devices 110a to 110d are connected in series, 2048 bits may be stored in one, and up to 8192 bits may be stored when all four memory devices are used.

For example, if 2048 bits or less are applied to the FFT output signal, only the first storage device 110a is used, and when the FFT output signal is larger than 2048 and less than 4096 bits, the first and second are stored together.

The stored FFT output signals are complex-multiplied by the conjugate signal multiplied by the signal before input to the storage device from the corresponding conjugate complex multipliers 130a to 130c to obtain a correlation value. By inputting and detecting the maximum value, the FFT mode is detected. Although not shown, it is desirable to accumulate correlation values by placing an accumulator before the maximum value detector 140 to increase the accuracy of detection, and obtain the maximum value of the accumulated correlation values from the maximum value detector 140.

For example, when the received signal has a symbol size of 4096 bits, a correlation value is obtained through the second conjugate complex multiplier 130b, and the correlation value is detected by the maximum value detection unit 140 as a maximum value. . The FFT mode detector according to the present embodiment makes it possible to quickly detect the FFT mode by enabling the FFT mode to be detected at once for all modes using a memory device connected in series with respect to the same signal.

Although four memory devices are used in this embodiment, different numbers of memory devices may be used depending on the memory size of the memory device and the size of a symbol of a signal that can be received and serviced. In addition, although three conjugate complex multipliers 130a to 130c are employed to detect 2k, 4k, and 8k modes, different complex conjugate multipliers may be used depending on the type of mode. Meanwhile, the outputs of the conjugate complex multipliers 130a to 130c are input to the guard mode detector 200.

The guard mode detector 200 according to the present embodiment includes a conjugate complex multiplier 210, a MUX 220, first and second shift sum units, first and second absolute value calculators 260a and 260b, and a multiplier 270. , An adder and subtractor 280, and a guard mode detector 290.

The conjugate complex multiplier 210 performs a complex conjugate multiplication of the signals input to the memory devices 110a through 110d to obtain a correlation value. The MUX 220 previously uses the conjugate complex multiplier 130a to FFT mode detector. The outputs of 130c) are input and multiplexed.

The first and second moving sum units move the sum of the output value of the MUX 220 and the output of the conjugate complex multiplier 210. For this shift sum, the first and second shift sums include delay units 230a and 230b, subtractors 240a and 240b, and adders 250a and 250b, respectively. This shift sum eventually adds the correlation values of the samples included in the signal sequentially. Here, 1/32 of the delay units 230a and 230b means to delay by the minimum mode length of the guard mode. Accordingly, shift sums as many as the number of samples in the 1/32 guard mode are obtained.

The first and second absolute value calculators 260a and 260b obtain absolute values of output values of the first and second moving sums. Multiplier 270 is the (1/2) ^1/2, and multiplying the second absolute value calculating unit (260b), adder-subtracter 280 is added or subtracted to the output value of the output value and the multiplier 270 of the first absolute value calculation unit (260a) Subtract Here, the output value of the multiplier 270 is subtracted from the output value of the first absolute value calculator 260a.

The guard mode detector 290 detects a corresponding guard mode according to the output value of the adder-subtracter 280.

The guard mode detector 200 having such a configuration detects the guard mode through a so-called normalized maximum correlation (NMC) method. In more detail, the NMC method is as follows.

.........식(1)

......... Equation (1)

The denominator in Equation (1) means the moving sum of the correlation values for the guard interval of its signal, and the numerator means the moving sum of the correlation values between the part and the guard interval after subtracting the FFT length. That is, referring to FIG. 2, the sum of the correlation values of the front guard period itself becomes the denominator, and the sum of the correlation values of the guard part of the front part and the end of the FFT period becomes the denominator. .

The portion for obtaining the numerator and denominator value is the shift sum portion of FIG. 3, the first shift sum portion at the rear end of the MUX 220 calculates the numerator portion, and the second shift sum at the rear end of the conjugate complex multiplication unit 210 calculates the denominator portion. .

In addition, the absolute value calculators 260a and 260b obtain absolute absolute values of the numerator and denominator by taking absolute values of the calculated numerator and denominator values, and the multiplier 270 determines the absolute value of the denominator (1/2) ^1. By multiplying the value of ^{/ 2, the} absolute value of (1/2) ^1/2 × denominator is obtained, and the subtractor 280 subtracts the (1/2) ^1/2 × denominator absolute value from the absolute value of the numerator. Finally, the guard mode detector 290 compares whether the result is greater than or equal to zero. The calculation in this guard mode detector 200 is, after all, the same as determining whether x (n) ≥ (1/2) ^1/2 of equation (1) is satisfied.

In this way, it is determined whether the value of Equation (1) is greater than a predetermined value, for example, (1/2) ^1/2 , counting the number of samples corresponding to the large case, and determining which guard mode the number corresponds to the guard mode. Will be detected. Since there is an uncertainty in detecting such a guard mode at one time, the mode in which the largest number of times occurs by repeating a predetermined number of times, for example, about 10 times, is detected as the guard mode.

On the other hand, in the determination of which guard mode corresponds, the number of samples obtained does not exactly match the number of samples in the guard mode, so a criterion for mode determination is necessary. For example, three thresholds, i.e., the first threshold is the average number of samples in the 1/8 mode and 1/16 mode, the second threshold is the average number of samples in the 1/32 mode and 1/16 mode, and The third threshold value is set to the average number of samples in the 1/8 mode and the 1/4 mode, and first, the comparison is made with the first threshold value, and then the comparison with the second or third threshold value is performed. Guard mode is divided into 1/32 mode when the threshold is smaller, 1/16 mode when the threshold is larger, 1/8 mode when the threshold is smaller, and 1/4 mode when the threshold is smaller. Separate.

The guard mode detector according to the present embodiment can quickly detect the guard mode using the NMC method. That is, although the guard mode is detected for each mode in the related art, the guard mode detector of the present embodiment uses the correlation values from the FFT mode detector previously connected in series and the correlation values before being input into the storage device at one time through the NMC method. The guard mode can be detected. However, the number of NMC methods may be performed several times in order to increase the accuracy of detection. On the other hand, since the guard mode detector according to the present embodiment uses the NMC method, the memory device conventionally used for the guard mode detection is unnecessary. Therefore, in the mode detector implementation, it is possible to significantly reduce the size of the memory, that is, the hardware size, and consequently to reduce the size of the receiving device.

4 is a flowchart illustrating a process of detecting a guard mode using an NMC method according to an embodiment of the present invention.

Referring to FIG. 4, first, it is determined whether FFT mode detection is completed (S100). Since the guard mode depends on the size of the FFT mode, it is natural that FFT mode detection should be preceded first. Next, an initial value for each variable is set (S200).

Here, iter_n is the number of times to perform the NMC method, it is set to a predetermined number N. The predetermined number N is preferably about 10. Iteration is a variable for counting the number of times the NMC method is performed, and 32mode, 16mode, 8mode, and 4mode are variables for counting the number of modes detected through the NMC method, respectively.

When the initialization operation is completed, first, it is compared whether the number of times (iteration) performed is greater than the predetermined number of times (iter_n) (S300). The larger case is the case where all of the set number of times is performed, so the process proceeds to the final guard mode estimating step (S900) and the smaller case proceeds to the NMC calculation step (S400).

Here, in the guard mode estimating step (S900), the maximum value of 32mode, 16mode, 8mode, and 4mode is selected and estimated as the final guard mode. If there are two maximum values or if the first maximum value and the next maximum value are almost indistinguishable, it is preferable to perform the guard mode detection process again because the detection uncertainty is high.

As described above, in the NMC calculation step S400, the number of samples in which Equation (1) becomes a predetermined value, for example, (1/2) ^1/2, is counted. Next, the number of counted samples (Count_N) is greater than 0 is compared (S500). If less than or equal to 0, the detection is not performed and the number of executions is increased without determining the mode (S800).

If the number of samples is greater than zero, the number of samples is compared with the first threshold DecisionBoundary2 (S600). Here, the first threshold value is preferably set to an average value of the number of samples in the 1/8 mode and the 1/16 mode.

If it is smaller than the first threshold value, the process proceeds to step S620 in which the second threshold value is determined (DecisionBoundary1), and when larger, the process proceeds to step S630 in which it is compared with the third threshold value DecisionBoundary3. Here, the second threshold is preferably set to an average value of the number of samples in 1/16 mode and 1/32 mode, and the third threshold is set to an average value of the number of samples in 1/4 mode and 1/8 mode. It is preferable.

In the step S620 of comparing with the second threshold value, when it is not greater than the second threshold value, it is determined as 1/32 mode and the number of 32 modes is increased (S700). If greater than the second threshold, it is determined as 1/16 mode and the number of 16 modes is increased (S720). On the other hand, in the step of comparing with the third threshold value, if not greater than the third threshold value is determined as 1/8 mode and the number of 8mode is increased (S760). If greater than the third threshold, it is determined as 1/4 mode and the number of 4modes is increased (S740).

After the guard mode increase step as described above, the method proceeds to step S800 of increasing the number of iterations, and then proceeds to step S300 of comparing whether the number of times of iteration is greater than a predetermined number of times iter_n. Then, the mode detection process is continued until the set predetermined number of times is reached.

When the number of executions reaches a predetermined number N, the process proceeds to the guard mode estimating step (S900), and the maximum value is found by comparing the 32-mode, 16-mode, 8-mode, and 4-mode variables counted up to that point, and the corresponding mode is estimated as the final guard mode. do.

So far, the present invention has been described with reference to the embodiments shown in the drawings, which are merely exemplary, and those skilled in the art will understand that various modifications and equivalent other embodiments are possible therefrom. . Therefore, the true technical protection scope of the present invention will be defined by the technical spirit of the appended claims.

1 is a block diagram of an OFDM receiver using an NMC method according to an embodiment of the present invention.

2 is a structural diagram showing an OFDM symbol structure in a pre-FFT period.

3 is a block diagram illustrating an FFT mode detector and a guard mode detector of FIG. 1.

4 is a flowchart illustrating a process of detecting a guard mode using an NMC method according to an embodiment of the present invention.

Claims (17)

An FFT mode detector for detecting an FFT mode using at least two storage devices connected in series; And A mode detector comprising a guard mode detector for detecting a guard mode using a normalized maximum correlation method. According to claim 1, The FFT mode detector includes four memory devices connected in series; And The maximum value is detected by receiving a correlation value between the OFDM symbol before the input into the storage device and the output of the first storage device, the correlation value of the output of the second storage device, and the correlation value of the fourth storage device output. And a maximum value detector. According to claim 1, The NMC method, The absolute value of a moving sum of the correlation values obtained using the storage device is used as a numerator, and the absolute value of the moving sum of the correlation value of the OFDM symbol itself before being inputted to the storage device is used as a denominator. And detecting a guard mode by counting the number of samples whose fractional value is equal to or greater than a predetermined threshold value. The method of claim 3, In order to implement the NMC method, The guard mode detector may include: a MUX for receiving multiplexed correlation values obtained using the memory device and outputting the multiplexed values; A first moving sum unit for moving the MUX output; A second shift sum unit for shifting a correlation value of the OFDM symbol itself before input to the storage device; First and second absolute value calculators for calculating absolute values of output values of the first and second moving sums, respectively; A multiplier for multiplying the output value of the second absolute value calculator by a predetermined value; An adder / subtracter for adding or subtracting an output value of the multiplier to the output value of the first absolute value calculator; And And a guard mode detector for determining a guard mode according to the result value of the adder and subtractor. According to claim 1, And the guard mode detector detects the guard mode by repeatedly applying a predetermined number of NMC methods. Performing FFT mode detection, guard mode detection, symbol time offset (STO), fractional carrier frequency offset (FCFO) estimation, and symbol frequency offset (SFO) estimation on an OFDM signal converted to a baseband digital signal through a receiver Pre-FFT portion; A CP remover for removing a cyclic prefix (CP) for the OFDM symbol; An FFT unit performing an FFT on the output signal of the CP removing unit; A distributed pilot extractor for extracting a distributed pilot with respect to the output signal of the FFT unit; A channel estimating apparatus for estimating a channel using the distributed pilot with respect to the output signal of the FFT unit; A demapper which demaps an output signal of the channel estimator; And And a channel decoder configured to perform channel decoding on the output signal of the demapper. And the guard mode detection is detected through a guard mode detector using an NMC method. The method according to claim 6, The NMC method, The absolute value of a moving sum of the correlation values obtained using the storage device is used as a numerator, and the absolute value of the moving sum of the correlation value of the OFDM symbol itself before being inputted to the storage device is used as a denominator. And detecting a guard mode by counting the number of samples whose fractional value is equal to or greater than a predetermined threshold. The method of claim 7, wherein In order to implement the NMC method, The guard mode detector may include: a MUX for receiving multiplexed correlation values obtained using the memory device and outputting the multiplexed values; A first moving sum unit for moving the MUX output; A second shift sum unit for shifting a correlation value of the OFDM symbol itself before input to the storage device; First and second absolute value calculators for calculating absolute values of output values of the first and second moving sums, respectively; A multiplier for multiplying an output value of the second absolute value calculator by a predetermined value; An adder / subtracter for adding or subtracting an output value of the multiplier to an output value of the first absolute value calculator; And And a guard mode detector for determining a guard mode according to a result value of the adder and subtractor. Determining whether to terminate FFT mode detection; Setting an initial value for guard mode detection; Determining the number of detections; Counting samples via NMC method; Determining a guard mode based on the counted number of samples and counting each guard mode. The method of claim 9, The NMC method uses an absolute value of a moving sum of correlation values obtained by using a storage device of an FFT mode detector as a numerator, and shifts a correlation value of an OFDM symbol itself before input to the storage device. A guard mode detection method, characterized in that the guard mode is detected by counting the number of samples whose absolute value is greater than or equal to a predetermined threshold with the absolute value of. The method of claim 9, If the FFT mode detection is not terminated in the step of determining whether to terminate the FFT mode detection, the FFT mode detection is performed. In the initial value setting step, the guard mode detection method, characterized in that for setting the number of times of detection, the number of each mode to zero. The method of claim 9, In the detecting number determination step, The guard mode, characterized in that it is determined whether or not greater than the number of detections set in the initial value setting step, the guard mode having the maximum value among the number of guard modes counted up to that time is estimated as the guard mode of the received OFDM symbol. Detection method. The method of claim 12, In the initial value setting step, the detection frequency is two or more times, And if there is no difference between the maximum value and the second maximum value among the count values of the guard modes or if there is only a predetermined number of differences, the guard mode detection method is started again. The method of claim 9, Counting the guard mode is Determining whether the number of samples is greater than zero; Comparing the number of samples with a first threshold; Comparing the number of samples with a second or third threshold; And And increasing the number of corresponding guard modes based on the result of comparing the second or third threshold values. 15. The method of claim 14, In the detecting number determination step, If it is less than or equal to the number of detections set in the initial value setting step, Proceeding to the step of counting the sample through the NMC method, In the step of determining whether the number of samples is greater than zero, if the number of samples is not greater than zero, the number of executions is increased and the process returns to the step of determining the number of detections. In the step of comparing the number of samples with the first threshold value, the second mode is compared with the second threshold value if not large, and if the comparison proceeds to the third threshold value if the guard mode, characterized in that Way. The method of claim 15, The first threshold is the average number of samples in the first mode and the second mode, The second threshold is the average number of samples in the third mode and the second mode, The third threshold is a guard mode detection method, characterized in that the average number of samples of the first mode and the fourth mode. The method of claim 16, In the step of comparing with the second threshold value, if it is not greater than the second threshold value, it is determined as the third mode and the number of third modes is increased, and if it is larger than the second threshold value, the second mode value is determined. To increase the number of second modes, In the comparing with the third threshold value, when it is not greater than the third threshold value, the first mode is determined and the number of first modes is increased, and when the value is larger than the third threshold value, the fourth mode value is determined. Increase the number of fourth modes, And increasing the number of executions after the mode increasing step, and returning to the step of determining the number of detections.

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